1. Field of the Invention
The present invention relates to a method and an apparatus for injection molding a semi-molten metal into a mold cavity to produce a thick molded article
2. Prior Art
As a method of producing a metal molded article having better internal quality than that made by die-casting, a semi-molten metal injection molding method wherein a molten metal (magnesium alloy), in a semi-melting state at a temperature of not more than a liquidus temperature of the metal material, is injected into a cavity of a mold has conventionally been known, as disclosed in Japanese Patent Publication JP-B 2-15620 (1990), corresponding to U.S. Pat. No. 4,694,882. Since the method for injection molding a semi-molten metal makes it possible to mold the metal at a relatively low temperature, the useful life of molds can be made longer than that of a mold used in die-casting, and moreover the high molding accuracy can be maintained for a long time of repeated moldings.
When molding a thick-wall metallic article having a thickness of not less than 5.0 mm in a product portion corresponding to a cavity by injection molding, a die casting method is apt to cause disturbance in the molten metal flow, during filling the cavity with the molten metal, which leads to gas entrapment and lower internal quality. Therefore, the injection molding method capable of injecting the semi-molten metal in a state of a laminar flow is more suitable than the die casting because of its high viscosity in the presence of a solid phase in the melt.
However, even if the semi-molten metal injection molding method is applied to the thick-wall article, filling and gas defects, shrinkage cavity, etc., cannot be avoided in the thick-wall parts of the molded article when setting the same molding conditions as in the case of producing a conventional relatively thin-wall molded article. Thus, for the injection molding process it is difficult to mold the thick molded article with high quality.
An object of the present invention is to provide a method of injection molding a semi-molten metal to a thick molded article with high quality by properly setting the molding conditions, thereby, obtaining a thick product free from internal defects,
The present invention is intended to produce a thick molded article, having thickness of not less than 5.0 mm in an are of 50% or more of a product portion corresponding to the cavity, is produced by injection molding a semi-molten metal of a metal material, at a temperature of not more than a liquidus temperature of the metal material, into a cavity of a mold through a product gate. To this end, the present invention, the solid fraction in the molten metal is set to 10% or more.
A solid fraction in the semi-molten melt lower than 10% causes the thick product to have internal defects such as gas defects in the thick portion. The high solid fraction can be easily adjusted by the temperature of the semi-molten metal held in the injector.
Preferably, the solid fraction of the semi-molten metal to be injected may be set within a range of 40 to 80%. As the solid fraction is higher than 40% the thick product may reduce in internal defects, while the solid fraction larger than 80% causes a reduction in fluidity in the semi-molten metal, resulting in filling defects into the mold cavity. The solid fraction in the above defined range can most effectively prevents both the filling defects and internal defects to obtain a high quality of the thick molded article.
In the invention, a sectional area of a product gate portion of the thick molded article corresponding to the product gate may preferably be set to not less than 0.1 times a sectional area of the product portion in the vicinity of the product gate. Each of the sectional areas means an area in a sectional plane perpendicular to a flow direction of the semi-molten melt.
The sectional area of a product gate portion smaller than 0.1 times a sectional area of the product portion, disturbance is liable to occur in the semi-molten melt flow into the cavity from the gate, which leads to entrapment of gas babbles in the metal. Therefore, the sectional area of a product gate portion of the thick molded article corresponding to the product gate is set to not less than 0.1 times a sectional area in the vicinity of the product gate in the product portion.
In the invention, the velocity Vg (mm/s) of the semi-molten metal when passing through the product gate, a sectional area Sg (mm2) of the product gate portion of the thick molded article and a volume Vp (mm3) of the product portion are set so as to satisfy the following relationships:
Vgxe2x89xa68.0xc3x97104;
and,
Vgxc3x97Sg/Vpxe2x89xa710.
That is, the product gate velocity Vg mm/s of the semi-molten melt is set to not more than 8.0xc3x97104 because the velocity Vg mm/s of larger than 8.0xc3x97104 is liable to cause disturbance in the metal flow. When the product gate velocity Vg mm/s of the semi-molten metal is too small and Vgxc3x97Sg/Vp becomes smaller than 10, the semi-molten melt is solidified until the semi-molten melt is perfectly filled to the cavity, resulting in filling defects in the molded product. Therefore, the molding conditions are required to satisfy the relationships of Vgxc3x97Sg/Vpxe2x89xa710. This feature in the invention makes it possible to obtain the thick molded article of higher quality, effectively preventing filling defects in the molded products.
In the invention, at least one product gate is connected to a portion of the cavity corresponding to the maximum thickness portion of the product portion of the thick molded article, continuing to apply a pressure to the maximum thickness portion to be finally solidified in the product portion until the maximum thickness portion is solidified. Therefore, shrinkage pores in the metal can be prevented from forming in the product portion having maximum thickness.
In the invention, a mold temperature in the vicinity of the product gate is set to be higher by 50xc2x0 C. or more than that of the cavity.
This construction makes it possible to prevent the semi-molten melt filled in the product gate from solidifying earlier than the semi-molten melt filled in the cavity, and to apply a pressure securely to the semi-molten melt filled in the cavity. Consequently, it is possible to securely inhibit the shrinkage cavity from forming at the production portion of the thick molded article.
In the invention, the heating means is provided in the vicinity of the product gate, and the mold temperature in the vicinity of the product gate is set to be higher by 50xc2x0 C. or more than that of the cavity by using the heating means. This construction makes it possible to easily control the mold temperature in the vicinity of the product gate to a temperature higher than that of the cavity.
In the invention, the solid fraction of the semi-molten melt filled in the product gate is set to a value which is 10% higher than that of the semi-molten melt filled in the 7.
According to this invention, since the semi-molten melt filled in the product gate is solidified earlier than that filled in the cavity, it is possible to effectively inhibit shrinkage cavity from forming at the product portion of the thick molded article.
The invention is an invention of a semi-molten metal injection molding apparatus of producing a thick molded article whose thickness is not less than 5.0 mm in the portion of not less than 50% of a product portion corresponding to the cavity, by injecting a semi-molten melt of a metal material, in a semi-melting state at a temperature of not more than a liquidus temperature of the metal material, into a cavity of a mold through a product gate.
In this invention, the solid fraction of the semi-molten melt is set to not less than 10%. In the invention, the solid fraction of the semi-molten melt is set within a range of 40 to 80%.
The sectional area of a product gate portion of the thick molded article corresponding to the product gate is set to not less than 0.1 times a sectional area in the vicinity of the product gate in the product portion.
In the invention, wherein a product gate velocity Vg mm/s of the semi-molten melt, a sectional area Sg mm2 of the product gate portion of the thick molded article and a volume Vp mm3 of the product portion are set so as to satisfy the following relationships:
Vgxe2x89xa68.0xc3x97104
and
Vgxc3x97Sg/Vpxe2x89xa710.
In the invention, at least one product gate is connected with a portion corresponding to the maximum thickness portion of the product portion of the thick molded article in the cavity.
In the invention, the mold temperature in the vicinity of the product gate is set to be higher by 50xc2x0 C. or more than that of the cavity In the invention, the heating means is provided in the vicinity of the product gate, and the mold temperature in the vicinity of the product gate is set to be higher by 50xc2x0 C. or more than that of the cavity by using the heating.
In the invention, the solid fraction of the semi-molten melt filled in the product gate is set to a value which is 10% higher than that of the semi-molten melt filled in the cavity.